As an electrode for electrochemically measuring the concentration of an ion such as an ammonium ion with setting environmental water such as river water, lake water, or seawater, water or wastewater, industrial wastewater, or the like as a sample, a liquid membrane type ion electrode is known (Patent Literature 1).
As a responsive membrane of this sort of ion electrode, a membrane having properties of a semipermeable membrane is used; however, in the case where ion concentration of the total of solutes of internal liquid is far higher than ion concentration of a sample, due to a difference in osmotic pressure between the sample and the internal liquid, water molecules on the sample side permeate the internal liquid side, and therefore an indicated value may vary, or in some cases, as illustrated in FIG. 7, the responsive membrane may expand and finally explode. For example, in the case where a difference between the ion concentration of the salt components of the internal liquid of the ion electrode (internal liquid used for an ion electrode or a reference electrode is considered to almost consist of salt components) and the ion concentration of the total of solutes of the sample is 6.55 M, a calculational difference in osmotic pressure is as much as 16.4 MPa.
On the other hand, in the case of decreasing the ion concentration of the salt components of the internal liquid, there is a problem that making the concentration of a halide ion contained in the internal liquid of the ion electrode coincide with the concentration of a halide ion contained in internal liquid of a reference electrode (a salt component used for such internal liquids is generally a halide metal salt, for example, KCl), which is generally performed, is impossible, and therefore a so-called isothermal point at which a membrane potential does not change even as temperature is changed shifts from a measurable range to increase a measurement error associated with a temperature change.